Pyrrole-tailed imidazolium surface-active monomers: aggregation properties in aqueous solution and polymerization behavior

Abstract

Pyrrole-functionalized imidazolium surfactants PyCnMImBr with different alkyl chain lengths (n = 8, 10, 12) were synthesized and their aggregation properties in water were investigated by tensiometry and conductimetry. These studies show that PyCnMImBr surfactants present lower critical micellar concentration than their non-functionalized analogues due to (i) an increased hydrophobicity resulting from the extension of the alkyl chain length upon incorporation of pyrrole (ii) the formation of attractive π-π interactions among pyrrole moieties. The areas occupied by PyCnMImBr molecules are always higher than those of CnMImBr surfactants showing looser molecular arrangement at the air-water interface due to steric hindrance of pyrrole moieties. 1H nuclear magnetic resonance measurements have been used to shed light on the structure of the aggregates formed. In all cases, pyrrole is located inside the micellar core but for the shortest alkyl chain length, it tends to intercalate between the alkyl chains. Micelles obtained with PyC12MImBr were further used as nano-reactor to prepare water-stable polypyrrole nanoparticles. For this, we conduct the chemical polymerization of pyrrole inside the micelles using FeCl3 as oxidizing agent. Polymerization kinetics were followed by UV-spectroscopy. Combined with Infrared (IR) and Dynamic Light Scattering (DLS) measurements, we confirmed the formation of polypyrrole particles of about 4 nm that were water-stable thanks to the presence of cationic imidazolium functions on their surface.